ROMULUS, Mich. (AP) -- Hundreds of people were advised to remain away from their homes Thursday as a fire at a suburban Detroit chemical plant continued to send smoke over their neighborhoods more than 36 hours after it started with a thunderous explosion.
''The fire is still burning, it's smoldering,'' Wayne Fire Chief Tim Reynolds told reporters at a news briefing. ''We're asking people to be patient.''
Reynolds said preliminary tests by the U.S. Environmental Protection Agency show no significant air contamination, but authorities were waiting for health officials to review data and give residents the OK to return to the area.
''We want to be absolutely sure that it's safe,'' he said.
Witnesses said they heard several loud explosions shortly before 10 p.m. EDT Tuesday at the E.Q. Resource Recovery Inc. plant in Romulus, then saw flames and smoke shoot into the sky. The facility recycles chemicals such as airplane deicing fluid and industrial paint solvents.
Mayor Alan R. Lambert said one tank exploded, then set off explosions in others at the plant. Uncertainty about the chemicals involved and the intensity of the fire initially kept firefighters from getting too close to the blaze.
By dawn Wednesday, the fire had died down, and crews moved in to put water and foam on the remaining flames. But hotspots continued to burn Thursday morning.
''There's a roof collapse over some drums that the fire department can't get to until we get some heavy equipment in there,'' Reynolds said. ''When they lift that roof off, the oxygen can get in there and it could flare up again.''
At least 32 people were treated at hospitals, Oakwood Healthcare System spokesman Tom Worobec said. Most complained of a burning sensation in their mouths or difficulty breathing. Some firefighters also suffered heat-related illnesses as they fought the fire.
Eight employees were working at the plant before the explosions, but they evacuated when an emergency horn sounded, and none of them requested medical treatment, company spokesman Dan Gilbert said.
''They really couldn't tell us anything real concrete that would tell us what caused this,'' Gilbert said. ''As soon as the situation stabilizes, we're going to start an investigation.''
Romulus police evacuated homes in a one-half-mile radius, while officials in neighboring Wayne initially cleared out about 1,000 homes in an area under the path of the fire's smoke plume. About 150 homes in Romulus and 750 homes in Wayne remained evacuated Thursday morning.
Many residents took refuge in two schools, where the American Red Cross provided cots and food. Officials said many others were staying with relatives elsewhere.
Wayne City Manager John Zech told evacuees at Wayne Memorial High School on Wednesday that they had the option to return to their homes, but fire officials recommended that they stay away until environmental tests were completed.
''The situation at the site is still unstable, so we do not have the all-clear sign as of yet,'' Zech said Wednesday. ''There are some unknowns, and the severity of the unknowns we're not sure of.''
Hazardous materials officials had tested the air quality and found no danger present, Romulus Public Safety Director Chief Charles Kirby said. EPA workers conducted more tests Wednesday.
Among the lingering concerns were possible dangers from the blanket of ash the fire had deposited on homes, cars and sidewalks in the evacuated area.
''We don't want anybody exposed to anything that would harm them,'' Zech said. ''If the dogs are walking on it, or you are, or your children, then you're going to be tracking it in your house, and we're concerned about that.''
Only a few dozen people remained at Wayne Memorial by Wednesday afternoon. But Imogene Williams, 80, said she wouldn't take any chances by returning home before officials said it was OK.
''If they say it's necessary to stay another night, we'll have to,'' Williams said. ''I just pray to God to protect my home.''